The Digestive System. Chapter 15

Transcription

1 The Digestive System Chapter 15

2 Introduction Digestion refers to the mechanical and chemical breakdown of food so the nutrients can be absorbed by cells Carried out by the digestive system Consists of the alimentary canal, leading from the mouth to anus and several accessory organs whose secretions aid the processes of digestion.

3 Digestion Chemical digestion complex molecules are broken down into smaller molecules Mechanical digestion physical movement of food that breaks it down into smaller pieces. Ex: chewing, grinding, mashing Chewing is called mastication

4 Alimentary Canal A muscular tube about 9 meters long that passes through the body s ventral cavity Movements of the alimentary canal Mixing movements occur when smooth muscles contract rhythmically in a small section of the tube Propelling movements a wavelike motion called peristalsis Peristalsis caused by contraction behind a mass of food as relaxation allows the mass to enter the next segment of the tube

5 Alimentary Canal

6 4 Layers of Alimentary 1) Mucosa: Canal Protects the tissues beneath it and carries out secretion and absorption Consists of epithelium, underlying CT, and a bit of muscle. 2) Submucosa: Nourishes surrounding tissue and carries away absorbed materials Consists of loose connective tissue, blood vessels, glands

7 4 Layers of Alimentary 3) Muscular Layer: Canal Layer which moves the tube Contains: two coats of smooth muscle tissue and nerves (plexus) Fibers of the inner coat surround the tube When they contract the tube s diameter decreases Fibers on the outer muscular coat run lengthwise When they contract the tube shortens 4) Serosa: Protects underlying tissues and secrete serous fluid

8 Alimentary Canal

9 Movements of the Alimentary Canal Mixing movements Occurs when smooth muscle in small units of the tube contract rhythmically Mixes food with digestive juices secreted by the mucosa Propelling movements Known as peristalsis

10 Mouth First portion of the alimentary canal Functions to receive food and begins mechanical digestion by mastication (chewing) Cheeks and Lips Cheeks form the lateral walls of the mouth Lips are highly mobile structures that surround the mouth opening Are highly sensitive and help to judge the temperature and texture of food

11 Mouth

12 Mouth Tongue A thick, muscular organ covered by mucous membrane and housing taste buds within papillae Attached to the floor of the mouth by the frenulum Palate The palate forms the roof of the oral cavity and has an anterior hard palate and posterior soft palate The soft palate and uvula function to close off the nasal cavity during swallowing Associated with the palate in the back of the mouth are palatine tonsils Help protect the body against infection

13 Tongue/Palate

14 Mouth Teeth Two sets of teeth develop in sockets within the maxillary and mandibular bones The 20 primary teeth are shed in the order they appeared and are replaced by 32 secondary teeth. Through the actions of chewing, teeth break food into smaller pieces, beginning mechanical digestion

15 Teeth

16 Teeth Different teeth are adapted to handle food in different ways and include: Incisors Cuspids Bicuspids Molars Each tooth consists of a crown and a root Each tooth is made of enamel, dentin, pulp, cementum, nerves and blood vessels

17 Anatomy of a Tooth

18 Mouth Salivary Glands Secrete saliva, which moistens food particles, binds them together, allows tasting, helps to cleanse the mouth and teeth and begins carbohydrate digestion. Receive parasympathetic stimulation that triggers the production of a large volume of saliva at the sight or smell of food Contain 2 cells Serous cells: produce amylase (breaks down carbs) Mucus cells: produce mucus to bind to food and lubricate during swallowing.

19 Salivary Glands

20 Pharynx and Esophagus Pharynx A cavity lying behind the mouth Connects the nasal and oral cavities with the larynx and esophagus Is divided into a nasopharynx (top portion), oropharynx (middle portion) and laryngopharynx (bottom portion) Swallowing Mechanism: Swallowing reflexes can be divided into 3 stages: Food is mixed with saliva and voluntarily forced into the pharynx with the tongue Sensory receptors in the pharynx sense food, which triggers swallowing reflexes. Peristalsis transports the food in the esophagus to the stomach

21 Pharynx and Esophagus Esophagus A muscular, straight, collapsible passageway leading to the stomach Lined with mucous glands that moisten and lubricate the inner lining of the tube. The lower esophageal sphincter helps to prevent regurgitation of the stomach contents into the esophagus.

22 Heartburn Regurgitation of stomach acid into the esophagus.

23 A J-Shaped muscular organ that receives and mixes food with digestive juices, and propels food to the small intestine A pyloric sphincter controls release of food from the stomach into the small intestine. Stomach

24 Stomach Has about a 1 liter capacity Contains rugae which allows it to expand Divided into 4 regions Cardiac Fundic Body Pyloric

25 Stomach Mixing and Emptying Actions Following a meal, mixing actions of the stomach turn the food into chyme and pass it toward the pyloric region using peristaltic waves The rate at which the stomach empties depends on the fluidity of the chyme and the type of food.

26 Stomach Ulcers Breaking down of mucosal lining.

27 Produces pancreatic juice that aids digestion Pancreatic juice contains enzymes that digest carbohydrates, fats, proteins and nucleic acids. Protein-digesting enzymes are released in an inactive form and are activated upon reaching the small intestine. Pancreas

28 Liver Reddish-brown structure located in the upper-right quadrant of the abdominal cavity The body s largest internal organ Divided into left and right lobes Has many functions: Metabolism of carbohydrates, lipids and proteins Stores glycogen, vitamins A,D, and B12, iron, and blood Filters blood, removing damaged red blood cells and foreign substances, and removes toxins Secretes bile during digestion

29 Liver

30 Bile A yellowish-green liquid that includes water, bile salts, bile pigments, cholestorol and electrolytes. Bile pigments are breakdown products from red blood cells Only bile salts have a digestive function Bile salts emulsify fats and aid in the absorption of fatty acids, cholesterol and certain vitamins

31 Gallbladder Pear-shaped sac lying on the interior surface of the liver Contains the bile duct that leads to the duodenum A sphincter muscle controls the release of bile from the common bile duct Bile does not normally enter the duodenum until the gall bladder is stimulated to contract.

32 Gallbladder

33 Small Intestine Receives secretions from the pancreas and liver Completes digestion of the nutrients in chyme Absorbs the products of digestion Transports the remaining residues to the large intestine Consists of the duodenum, jejunum and ileum Duodenum Shortest and most fixed portion of the small intestine (the rest is mobile and lies free in the peritoneal cavity) Small intestine is suspended from the posterior abdominal wall by a double-layered fold of peritoneum called mesentery. Inner wall of the small intestine is lined with intestinal villi, which greatly increase the surface area available for absorption and aid in mixing actions.

34 Small Intestine

35 Small Intestine Absorption in the small intestine: Major site of absorption in the alimentary canal Movements of the small intestine: Small intestine moves chyme by peristalsis

36 Large Intestine Absorbs water and electrolytes and forms and stores feces Parts of the Large intestine Cecum pouch at the beginning of the large intestine Colon ascending, transverse, descending and sigmoid regions Rectum Anal Canal Opens to the outside as the anus Is guarded by an involuntary internal anal sphincter and a voluntary external anal sphincter muscle.

37 Large Intestine

38 Large Intestine Functions of the Large Intestine Does not digest or absorb nutrients Secretes mucus Absorbs water and electrolytes Movements of the Large Intestine Similar to those of the small intestine Peristaltic waves happen only two or three times during the day Defecation is stimulated by a defecation reflex that forces feces into the rectum where they can be expelled. Feces Composed of undigested material, water, electrolytes, mucus and bacteria.